1. Field of the Invention
This invention relates to electrographic printheads and more particularly to a method and apparatus having the capability of withstanding mechanical wear and dielectric build-up which resulted in poor print quality and/or shorts.
2. Description of the Prior Art
Printheads for electrographic printing are made from etched electrode circuit boards or arrays of wires that are bonded between two stainless steel support bars. The ends of the closely-spaced etched circuits or wires are exposed to create printing styli. Electrographic paper which is placed in contact with the surface of the printing head, contacts the exposed printing styli as it is dragged across the printhead. Digitally controlled printing voltages (0-500 volts D.C.) connected to the styli, electrically charge the surface of the electrographic paper to create a latent charge image. The latent charge image thus produced on the electrographic paper surface is then toned with electrostatically-charged pigment particles to produce a visible image. Typical prior art printheads are disclosed in U.S. Pat. Nos. 3,958,251 and 3,624,661.
Because the electrographic printhead is in close contact with the electrographic paper travelling at high speeds, several problems became evident. One problem arose when utilizing papers of various widths. When paper of narrow width was utilized, the wear on the electrode head created a stepped recess. When wider paper was utilized, the paper rested on the upper part of the step, whereas the inner part of the step was recessed. This created a gap between the electrode head and the paper thus preventing imaging. Another problem was the epoxy resin and substrate recession below the head bars. Still another problem was dielectric build-up on the styli and inter-stylus shorting, resulting in poor print quality.
In order to illustrate these problems, prior art FIGS. 1A, 1B, 2A, 2B and 3 are included.
Referring to FIGS. 1A and 1B the prior art printhead comprises a Teflon laminate substrate 1 having a plurality of copper styli 2 bonded thereon. This assembly is bonded to two stainless steel head bars 3 utilizing epoxy 4.
Referring now to FIGS. 2A and 2B, there is shown in cross section, a plan view and a side view (lengthwise) of a prior art electrographic printhead in contact with wide electrographic paper 5. The paper is flowing upward as indicated by the arrow 6. It can be seen that a recess (a) has worn into the electrode head from use of narrow paper 7 shown in dotted lines. The stepped recess (a) will prevent the imaging process.
Finally, referring to FIG. 3, a cross section of a prior art electrographic printhead is shown having styli 2 which have been sputtered to leave a recess 10.
What is required is an electrographic printhead having the following quality characteristics: (1) mechanical wear of the head bars should be minimized; (2) epoxy recession due to abrasive wear should be minimized; (3) electrical and mechanical erosion of styli should be improved; and (4) dielectric build-up on styli and inter-stylus shorting should be minimized.